1. Field Of The Invention
The invention relates to a new process for the production of squaric acid, in which squaric acid is isolated in high purity and with high yield, as well as to halogenated cyclobutenones as new intermediate products in the process.
2. Background Art
Squaric acid is an interesting intermediate product for the production of pharmaceutical agents, dyes (Angew. Chem., 20, (1966), p. 931) and herbicides (Swiss Patent No. 609,837). Various processes for the production of squaric acid are known from the literature.
Several of such processes start from hexachloro-1,3-butadiene, which is cyclized with sodium ethanolate to a chlorinated cyclobutene derivative. These intermediate products are hydrolyzed with sulfuric acid or other acids to squaric acid [Roedig, A., and Bernemann, P., Liebigs Ann. Chem., (1956), 600, p.1; Maahs, G., Liebigs Ann. Chem., (1965), 686, p. 55; Angew. Chemie, (1963), 75, p. 982; Uehara. A., and Tsuchiya, R., Sci. Rep. Kanazawa Univ., (1980), 25, p.83; Fan, R., et al., Chemical Abstracts, (1987), 106, 103798c]. Instead of sodium ethanolate, morpholine is also used [Maahs, G., and Hegenberg, P., Angew. Chemie, (1966), 78, p. 927; Schmidt, A. H., and Ried, W., Synthesis, (1978), p. 869; Gadek, T. R., et al., (1976), U.S. Pat. No. 4,104,308; Paine, A. J., Tetrahedron Letters, (1984), 25, p. 135]. The ring closure can also take place purely by thermal means [Mueller, W., (1976), German Patent No. 2,618,557; Schroeder, M., and Schaefer, W., (1976), German Patent No. 2,623,836; Maahs, G., and Rombusch, D., (1978), German OS 2,824,558; Rombusch, K., and Maahs. G., (1983), German Patent No. 3,314,431]. Disadvantages of all these processes are either modest yields or high expense (e.g., distillation with extreme reflux ratio) and the special safety measurements which are necessary in dealing with the carcinogenic feedstock hexachloro-1,3-butadiene.
According to another process [Bellus, D., et al., Helv. Chim. Acta, (1978), 61, p. 1784] squaric acid is obtained in 70 percent yield from the fungus metabolite moniliformin by bromation and hydrolysis. But moniliformin is present in nature only in small amounts and the known syntheses for it are expensive and produce only modest yields.
Another process, the electrochemical reductive tetramerization of carbon monoxide to squaric acid, requires a large expense for equipment and yields a product mixture from which the squaric acid can be isolated in pure form only with difficulty. [Silvestri, G., et al., Gazz. Chim. Itl., (1972), 102, p. 818; German OS 2,235,882; U.S. Pat. No. 4,461,681; U.S. Pat. No. 4,523,980; Fabre, P. L., et al., Bull. Soc. Chim. Fr., (1988), p. 933].